Specifically, it is known that direct solar illumination on an electromagnetic radiation collecting system gives rise to thermoelastic deformations of the latter; thereby rendering it inoperative during certain periods. It is even possible for direct electromagnetic radiation to cause irreversible degradations, or indeed the destruction of the instrument by thermal heating.
Currently, in order to attempt to alleviate this problem, multiple solutions have been developed. Firstly, it is possible to offset the sighting axis of the instrument when the solar flux illuminates the latter directly or when it is close to it. Another technology consists in momentarily obstructing the entrance of the instrument or any other zone of the instrument affected by the solar flux so as to protect it from direct illumination.
It is also known that it is possible to develop an “athermal” system the principle of which is that deformations are applied to the bearing structure of the collecting system so as to compensate for the deformations of the collecting elements under the effect of temperature variations.
Another technique for avoiding any degradation of the instruments consists in positioning solar filters at the entrance of the system or in another zone of the instrument.
Finally, in recent space instruments, a solar baffle system is commonly implemented. A solar baffle, positioned upstream of the collecting instruments with respect to the sun, is then intended to absorb and to diffuse the solar energy, notably by way of chicanes comprising metallic materials.
However, in all cases, losses or degradations of the images and unavailabilities of the collecting elements persist. This is extremely penalizing for current observation missions.
Furthermore, generally, one and the same structure supports at one and the same time the elements charged with absorbing the solar radiation and the mirrors or other collecting instruments. This necessarily involves deformations due to the significant temperature variations. Thus, even in the case where one and the same material, typically beryllium, is used for all the structures, thereby involving homothetic deformations of these structures and therefore no maladjustment of the optics, significant drawbacks will remain:
Firstly, though homothetic, the expansion of the instruments gives rise to an increase in the image spot with respect to its focal plan and therefore a maladjustment of the instrument (the pixels of the focal plan no longer observing the same scene, but an expanded scene) and a loss of resolution.
Next, in the case of geostationary satellites, the significant temperature variations occur over a period of about a day, this constituting too short a time to allow the temperature to become uniform in all the parts. To minimize this problem, it will commonly be sought to favour the diffusion of the solar heat in the equipment by lining the internal surface of the solar baffle with mirrors. But, in this way, the risk of degradation of the optics increases greatly on account of multiple reflections of the solar radiation within the equipment. Additionally, this principle increases the stray light in the instrument.
To summarize, none of the current technologies allows the continuous use of a collecting instrument pointing in a direction close to that of the sun without risk of degradation of the instrument or its performance, notably in terms of resolution and availability.